Alkanes
Structure & Nomenclature
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Alkanes Structure & Nomenclature
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Structure and Properties
An alkane is a hydrocarbon that contains only single bonds. The alkanes are the simplest and least reactive of organic compounds because they contain only hydrogen and sp3 hybridized carbon, and they have no reactive functional groups. Alkanes are saturated hydrocarbons because they have a maximum number of bonded hydrogens.
The table below shows the molecular and structural formulas for the first 6 unbranched members (straight chain) groups of the homologous alkane series. Notice how the molecular formulas increase by two hydrogens each time a carbon atom is added. Isomers of these compounds have the same molecular formulas but different structures.
The next table shows the structures and formulas of the first 20 unbranched alkanes. Here the structures are intentionally written as chains of CH2 (methylene) groups. terminated at each end by a hydrogen atom. If the molecule contains n carbon atoms, then it must contain (2n + 2) hydrogen atoms. The general formula for the homologous series of n-alkanes (or 'normal' alkanes) is therefore given by:
CnH2n+2
Branched alkanes (isopentane, isobutane, neopentane, etc.) as structural isomers of the alkane family, have the same general formula. Regarding nomenclature, the names methane, ethane. propane and butane all have historical roots. From pentane onward, alkanes are named using the Greek word for the number of carbon atoms, plus the suffix -ane to identify the compound as an alkane.
The physical properties of the alkanes follow the pattern exhibited by methane, and are consistent wit the alkane structure. An alkane molecule is held together entirely by covalent bonds. These bonds either join tow atoms of the same element (non-polar) or 2 atoms of differing elements and slightly differing electronegativities (slightly polar). These bonds are directed in a very symmetrical way, so that the slight polarities tend to cancel out. Thus an alkane molecule is either non-polar or weakly polar.
Furthermore, the van der Waals forces holding non-polar molecules together are weak and of very short range, and act only between the surfaces of molecules. Thus, the larger the molecule, the greater its surface area, and the stronger the intermolecular forces. The result, as evidenced in the table above, is that the boiling points and melting points -- which require overcoming the intermolecular forces holding together a liquid and a solid -- rise as the number of carbons increases. Except for very small alkanes, the boiling point rises 20 to 30 degrees for each carbon that is added to the chain.
In addition, a branched-chain isomer has a lower boiling point that a straight-chain isomer. The more numerous the branches, the lower the boiling point. This can be rationalized as follows. With branching, the shape of the molecule tends to approach that of a sphere. This serves to minimize the surface area. Thus the intermolecular forces become weaker, and are overcome at a lower temperature.
In agreement with the rule of thumb that "like dissolves like", the alkanes are soluble in non-polar solvents such as benzene, ether, and chloroform. They are insoluble in water and other highly polar solvents. Considered themselves as solvents, the liquid alkanes dissolve compounds of low polarity and do not dissolve compounds of high polarity.
The relative density increases with the size of the alkanes, and levels off at ~ 0.8. In general, to be more dense than water, a compound must contain a heavy atom like bromine or iodine or several atoms like chlorine.
IUPAC Naming System
The IUPAC naming system works consistently to name many different families of compounds. We will consider the naming of alkanes in detail, and later extend these rules to other kinds of compounds as we encounter them. The IUPAC system uses the longest chain of carbon atoms as the main chain, which is numbered to give the location of side chains. Four rules govern this process.
I. The Main Chain
Find the longest continuous chain of carbon atoms, and use the name of this chain as the base name of the compound.
The key word here is continuous. It does not matter whether the carbon skeleton is drawn in an extended straight-chain form or in branched form with many bends and turns. The simple fact is that the longest chain is rarely drawn in a straight line. All that matters is the number of carbons linked together in an uninterrupted sequence.
The groups attached to the main chain are called the substituents because they are substituted (in place of a hydrogen atom) on the main chain. When there are two longest chains of equal lengths, use the chain with the greater number of substituents as the main chain.
II. Numbering the Chain
To give the locations of the substituents, assign a number to each carbon atom on the chain, beginning with the end of the chain nearest a substituent. (This technique will minimize the numbers of substituted carbons).
III. Naming Alkyl Groups
Name the substituent groups attached to the longest chain as alkyl groups. Give the location of each alkyl group by the number of its attached carbon atom. Alkyl groups are named by adding the -ane suffix of the alkane name with -yl. E.G. Methane --> methyl; ethane --> ethyl, etc.
[Note: The propyl and butyl groups are simply unbranched 3 and 4 carbon alkyl groups. These groups are often named as 'n-propyl' and 'n-buytl' groups in order to distinguish them from other isomers].
The simple branched alkyl groups are usually known by common names. The isopropyl and isobutyl groups have a characteristic "iso" (CH3-CH3-CH) grouping, just as in isobutane. The names of the secondary-butyl (sec-butyl) and tertiary-butyl (tert-buytl or t-butyl) groups are based on the degree of alkyl substitution of the carbon atom attached to the main chain. In the sec-butyl group, the carbon atom bonded to the main chain is secondary, or bonded to 2 other carbon atoms. in the t-butyl group, it is tertiary, or bonded to three other carbon atoms. In both the n-butyl and the isobutyl group, the carbon atoms bonded ot he main chain are primary, or bonded to only one other carbon atom.
IV. Organizing Multiple Groups and Identical Groups
When two or more substituents are present, list them in alphabetical order. When two or more of the same alkyl substituent are present, use the prefixes di-, tri-, tetra-, penta-, hexa-, etc. to avoid repetition in naming identical groups.
V. Complex Substituents
Complex alkyl groups are named by a systematic method using the longest alkyl chain as the base alkyl group. The base alkyl group is numbered beginning with the carbon atom (the 'head carbon') bonded to the main chain. The substituents on the base alkyl group are listed with appropriate numbers, and parentheses used to offset the name of the complex alkyl group.
VI. Cycloalkane Nomenclature
Cycloalkanes are alkanes that contain a closed ring (alicyclic or aromatic) of three or more carbon atoms. They have a molecular formula given by:
CnH2n
For example, cyclohexane C6H12 is composed of a 6-membered non-aromatic carbon ring, with 2 hydrogen atoms bonded to each of the 6 carbon atoms in the ring. Cyclohexane is named by adding the prefix cyclo- to the name of the unbranched alkane with the same number of carbons as the ring. Substituent positions are specified by numbering the carbon atoms of the ring in the direction that gives the lowest number to he substituents at the first point of difference. When the ring contains fewer carbon atoms than an alkyl group attached to it, the compound is named as an alkane, and the ring is treated as a cycloalkyl constituent.
Conformational Analysis of Ethane
Conformations are different spatial arrangements of a molecule that are generated by rotation about single bonds. Conformational analysis is the study of how conformational factors affect the structure and properties of a molecular compounds. Ethane is the simplest compound that can have distinct conformations. The staggered and eclipsed conformations convert rapidly to each other by rotation about the C-C bond.
I. In the staggered conformation, each C-H bond of one carbon bisects an H-C-H angle of the other carbon.
II. In the eclipsed conformation, each C-H bond of one carbon is aligned with a C-H bond of another carbon.
The three most common ways of illustrating these structural features of ethane are the wedge-and-dash, sawhorse, and Newman projection drawings. These are shown below for the staggered and eclipsed conformations of ethane.
In a Newman projection, we sight down the C-C bond, and represent the forward carbon by a point and the rear carbon by a circle. Each carbon also has three other bonds that are placed symmetrically around it. The structural feature illustrated in the above figure is the spatial relationship between bonds on adjacent carbons. Each H-C-C-H unit in ethane is characterized by a torsion angle or dihedral angle, which can be viewed as either 1) the angle between the H-C-C plane and the C-C-H plane, or 2) the angle between the C-H bonds on the forward and rear carbon atoms. (These are the same by inspection).
The torsion angle is easily seen in Newman projections of ethane as the angle between C-H bonds of adjacent carbons. Eclipsed bonds are characterized by a torsion angle of 0 degrees, because the H atoms on the rear carbon are eclipsed or hidden by the H atoms on the forward C atom. When the torsion angle is approximately 60 degrees, we say that the spatial relationship is gauche. When the torsion angle is 180 degrees, we say that it is anti. Staggered conformations have only gauche or anti relationships on bonds between adjacent carbon atoms.
It should be noted here that any number of conformations are possible for ethane, since the angle between the H atoms on the forward and rear carbons can take on an infinite number of values. Thus, any intermediate conformations characterized by dihedral angle valaues of 0 and 60 degrees are referred to as skew conformations.
Conformational analysis is the study of the structural (mechanical) and thermodynamic stability of various structural conformations. Energetically speaking, the staggered conformation is 12 kJ / mol more stable than the eclipsed conformation.
Conformations of Propane
Propane is the 3-carbon alkane, with formula C3H8. The figure below shows a 3-dimensional representation of the staggered conformation of propane and a Newman projection looking down the line of one (either) of the C-C bonds.
The torsional energy of the eclipsed conformation of propane is about 13.8 kJ / mol. Note that the torsional strain resulting from eclipsing a C-H bond with a C - CH3 bond (propane) is only 1.2 kJ / mol more than the strain of eclipsing two C-H bonds (ethane).
Conformations of Butane
Butane is the four carbon alkane, with molecular formula C4H10. We refer to n-butane as a straight-chain alkane -- but the carbon backbone is not really straight. The angles between the carbon atoms are close to the tetrahedral angles of 109.5 degrees. Rotations about any of the C-C bonds are possible. Pictured below are the Newman projections, looking along the central C2 -C3 bond. (Note that we have defined the dihedral angle as the angle between the two end methyl groups).
Three of the butane conformations are given special names. When the methyl groups are pointed in the same direction (dihedral angle = 0) they are totally eclipsed. This is to be distinguished from the other eclipsed conformations, such as the ones at 120 degrees. At a dihedral angle of 60 degrees, the butane molecule is gauche (staggered) and the methyl groups are separated by 60 degrees. Another staggered conformation occurs at 180 degrees, with the methyl groups pointing in opposite directions. This conformation is called anti because the methyl groups are opposed to each other.
Torsional Energy of Butane
A graphical illustration comparing the torsional energies of the butane conformations as function of the dihedral angle is shown below (with the exception of the 360 degree totally eclipsed conformation). All the staggered conformations (anti and gauche) are lower in energy than any of the eclipsed conformations. The anti conformation is lowest in energy because it places the bulky methyl groups as far apart as possible.
The gauche conformations, with the methyl groups separated by just 60 degrees, are 3.8 kJ higher in energy than the anti conformation due to 'crowding'. I.E. the methyl groups are close enough together that their electron clouds begin to overlap and repel each other. This kind of interference is called steric strain or steric hindrance. Rotation of the totally eclipsed conformation by 60 degrees to a gauche conformation relieves most of the steric strain. The gauche conformation is a mere 3.8 kJ higher in energy than the most stable anti conformation.
Conformations of Higher Alkanes
These observations can be applied to other alkanes. We can predict that C-C single bonds will assume staggered conformations whenever possible to avoid eclipsing of the groups attached to them. Among the staggered conformations, the anti conformation is preferred because it has the lowest torsional energy.
Thus, the higher alkanes resemble butane in their preference for anti and gauche conformations about the C-C bonds. For example, the lowest energy conformation for any straight-chain alkane is the one with all the internal C-C bonds in their anti conformations. These anti conformations give the chain a zig-zag shape. At room temperature, many of the bonds undergo rotation, and the many of the molecules adopt gauche conformations, which create kinks in the zig-zag structure. Nevertheless, we often draw alkane chains in a zig-zag structure to represent the most stable (and thus most highly likely) arrangement.
